This proposal describes experiments designed to investigate the nature of oxidation-reductions of copper with respiratory cytochromes and related model compounds. The principal objective of the study is to assess the potential role of pi-unsaturated ligands in mediating electron transfer between metal ion redox centers in bilogical molecules. Impetus for the study derives from the recent structural characterization of heme a and our own research (J. Am. Chem. Soc., in press) on the spectra and electron transfer reactions of cobalt (III)- copper (I) dimers containing copper pi-coordinated to cobalt alkenoate ligands. Specific studies encompassed by the proposal are investigation of: (1) Cu(1) binding to heme a and the kinetic properties of Cu(I)-heme solutions, (2) internal electron transfer reactions between redox components of cytochrome c oxidase with emphasis upon detecting electron transfer involving copper and evidence for cooperative interactions between metal centers, (3) the effect of increasing pi-unsaturation in organic ligands upon Cu(I) reduction of transition metal oxidants, (4) the effect of Cu(I) in promoting electron transfer mediated by pi- conjugated ligands as a means of identifying transfer mechanisms.